The invention relates to an apparatus for the distortion-free back-and-forth transporting of a recording medium of an imaging machine such as a printer or copier between a counter-bearing, which receives the recording medium, and a plurality of adjacently arranged pressure rollers which press the recording medium against the counter-bearing.
In electrographic high-speed printers operating with continuous paper, stacking devices are used for stacking the printed continuous paper, such as are described, for example, in DE-C2-26 17 334. For stacking the recording medium on a stacking surface, a transporting device is arranged above the stacking surface, which transporting device comprises a transporting roller and a pressure roller between which the recording medium is transported by friction.
In order to achieve an even contact and guiding of the paper, it is also customary, instead of a continuous pressure roller, to arrange a plurality of individual pressure rollers adjacently, which press the recording medium against the transporting roller. Owing to the design, between these pressure rollers there is a gap in which the paper is not pressed on.
In order that the recording medium, consisting for example of prefolded continuous paper, can be placed securely on the delivery surface in the form of a stack, the recording medium must be fed to the delivery surface in an unstressed state. This means that the recording medium must roll along the stack below the feed device so that a zigzag stack can be formed automatically.
Electrographic printing devices must be constructed in such a way that they can be used to print on recording media of the widest variety of types, including thin and thick prefolded paper. There is thus the risk, in particular when using relatively thin prefolded paper, that the paper will form a groove in the region between the pressure rollers when passing through the feed device in the stacking device, which makes creasing in the fold and rolling of the paper more difficult. This groove in the paper, as illustrated in FIG. 2, is produced due to the fact that the paper is pushed together between the pressure rollers as a result of the contact pressure of the pressure rollers.
This groove formation can be avoided if the paper is drawn outwards at the sides and is thus made taut in the region of the pressure rollers. In order to achieve this, it has already been proposed to mount the outer pressure rollers with a slightly inclined axis in such a way that the pressure rollers exert an outward tensile force on the recording medium. This transverse force thus produced between the outer pressure rollers makes the recording medium taut in the region located between them.
Electrographic printing devices operate at very high printing speeds of 200 sheets per minute and higher. In order that no tearing of the recording medium occurs when the printing device has to be stopped, the printing device must not be stopped abruptly, but it is braked with a specific deceleration. The section of the recording medium which continues to be transported during this braking phase must subsequently be drawn back again for the next start-up of printing so that printing on the recording medium in the correct position is guaranteed.
During this drawing-back of the recording medium, however, the oblique setting of the outer pressure rollers now leads to the recording medium being pushed together between the pressure rollers, which can cause faulty stacking at the next start-up of printing. This is disadvantageous particularly in the case of frequent start-stop operation.
This problem also occurs in other printing devices which operate, for example, with large-format single sheets, and in which it is necessary to move the recording medium back and forth in a printing or transporting channel.
An electrographic printing device for two-sided printing of single sheets is known from U.S. Pat. No. 4,953,846. For this purpose, the single sheet is firstly printed in the printing station on the front, is fed via a separate return channel to a turning device, is turned there, is laterally offset by means of an obliquely set guide roller and is then printed on the back. A transporting device is used to transport the single sheets, which transporting device comprises a motor-driven transporting roller and a pressure roller which presses the single sheets resiliently against the transporting roller. In order that the pressure roller can follow the deflection of the single sheet, it is mounted in a bearing body so as to be horizontally displaceable with its axis of rotation loose at one end. In this way, the pressure roller is prevented from exerting guiding forces on the single sheets.
An object of the invention is to design a guide pressure roller for a recording medium, in friction-contact with the guide pressure rollers, of a printer or copier in such a way that, on the one hand, the angle of the axis of rotation of the guide pressure roller relative to the transporting direction of the recording medium changes in dependence on the direction of rotation of the guide pressure rollers and, in so doing, guide forces are exerted on the recording medium and, on the other hand, the guide pressure rollers is guided precisely.
A further object of the invention is to provide an apparatus for the back-and-forth transporting of a recording medium of a printer or copier between a counter-bearing, which receives the recording medium, and a plurality of adjacently arranged pressure rollers which press the recording medium against the counter-bearing, in which no distortion, for example by groove formation, occurs between the pressure rollers.